## Tidal effects on the radial velocity curve of HD 77581 (Vela X-1)

^{1} Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca, Mor. 62210, Mexico

e-mail: gloria@astro.unam.mx
^{2} Instituto de Astronomía, Universidad Nacional Autónoma de México, 04510, Mexico

e-mail: edmundo@astro.unam.mx
^{3} Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822 USA

e-mail: dmh@ifa.hawaii.edu

Received: 3 November 2011

Accepted: 9 January 2012

*Context.* The mass of the neutron star in Vela X-1 has been found to be larger than the canonical 1.5 *M*_{⊙}. This result relies on the assumption that the amplitude of the optical component’s measured radial velocity curve is not seriously affected by the interactions in the system.

*Aims.* Our aim is to explore the effect on the radial velocity curve caused by surface motions excited by tidal interactions.

*Methods.* We use a calculation from first principles that involves solving the equations of motion of a Lagrangian grid of surface elements. The velocities on the visible surface of the star are projected along the line-of-sight to the observer to obtain the Doppler shifts which are applied to the local line-profiles, which are then combined to obtain the absorption-line profile in the observer’s reference frame. The centroid of the line-profiles for different orbital phases is then measured and a simulated RV curve constructed. Models are run for the “standard” (*v* sin*i* = 116 km s^{-1}) and “slow” (56 km s^{-1}) supergiant rotation velocities.

*Results.* The surface velocity field is complex and includes fast, small-spatial scale structures. It leads to strong variability in the photospheric line profiles which, in turn, causes significant deviations from a Keplerian RV curve. The peak-to-peak amplitudes of model RV curves are in all cases larger than the amplitude of the orbital motion. Keplerian fits to RV curves obtained with the “standard” rotation velocity imply *m*_{ns} ≥ 1.7 *M*_{⊙}. However, a similar analysis of the “slow” rotational velocity models allows for *m*_{ns} ~ 1.5 *M*_{⊙}. Thus, the stellar rotation plays an important role in determining the characteristics of the perturbed RV curve.

*Conclusions.* Given the observational uncertainty in GP Vel’s projected rotation velocity and the strong perturbations seen in the published and the model RV curves, we are unable to rule out a small (~1.5 *M*_{⊙}) mass for the neutron star companion.

Key words: stars: neutron / binaries: spectroscopic / stars: rotation / stars: individual: HD 77581

*© ESO, 2012*